Code:
on @1:JOIN:#: { .timerlimit [ $+ [ $chan ] ] 1 5 /mode $chan +l $calc($nick($chan, 0) + 5) }
on @1:PART:#: { .timerlimit [ $+ [ $chan ] ] 1 5 /mode $chan +l $calc($nick($chan, 0) + 5) } 


You might want to add a channel name, so you dont set limits in all chans youre opped in.

on @1:JOIN:#channelname: { .timerlimit [ $+ [ $chan ] ] 1 5 /mode $chan +l $calc($nick($chan, 0) + 5) }
on @1:PART:#channelname: { .timerlimit [ $+ [ $chan ] ] 1 5 /mode $chan +l $calc($nick($chan, 0) + 5) }